Coated metal oxide particles with low dissolution rate, methods for preparing same and use thereof in electrochemical systems

ABSTRACT

Particles comprising a core and a coat covering at least part of the core surface. The core has more than 50% of an acidic metal oxide and the core coating is based on a polymer, preferably based on a soiled polymer with high electrochemical stability. The particle has a solubility rate (ts), in fixed time, of the metal oxide migrating towards the electrolyte, per cycle, which is less than 5 per 10000. The particles are obtained by mixing the polymer and a metal oxide, via dry process with addition of solvent. The electrodes constituting an electrode substrate at least partly coated with a mixture consisting of at least 40 of those particles have remarkable electrochemical properties, in particular regarding the lifetime of batteries in which they are incorporated.

FIELD OF THE INVENTION

The present invention relates to particles comprising a core based on anacidic metal oxide, the surface of which is at least partially coveredwith a polymer having an electrochemical stability of greater than 3.7volts.

The present invention also relates to processes for preparing a mixtureof these particles, especially incorporating steps of mixing theconstituents of the particles.

These particles have the advantage of having a low degree of solubilityof the metal oxide present in the core, even after having been subjectedto a large number of electrochemical cycles.

Among the many possible applications for the particle mixtures of theinvention, mention may be made of the covering of electrode supports.The electrodes thus obtained are particularly stable in operation andmay thus be advantageously used in high-performance electrochemicalsystems.

The electrodes and generators thus obtained are also a subject of thepresent invention, as are the processes for preparing the electrodes andgenerators of the invention.

BACKGROUND

The document “Study of Mn dissolution from LiMn₂O₄ spinel electrodesusing in situ total reflection X-ray fluorescence analysis andfluorescence XAFS technique” by Yasuko Terada in Journal of PowerSources 97-98 (2001), pages 420-422, thus demonstrates the phenomenon ofdissolution of LiMn₂O₄ oxides in storage batteries and the loss ofcapacity resulting therefrom.

The document “In situ XAFS study of the electrochemical deintercalationof Li from Li_(1-x)Mn_(2-y)Cr0₄” by Izumi Nakai et al. in Journal ofPower Sources 97-98 (2001), pages 412-414, also describes the phenomenonof dissolution of metal oxide and proposes to partially overcome theinstability of the structure by the partial substitution of Mn with Cr,Co or Ni. This technique proves to be complex and has drawbacks due tothe fact that it requires a novel synthesis and adjustment of the dopantsuch that the structure is electrochemically stable.

The document “Influence of the particle size on the electrochemicalproperties of lithium manganese oxide” by Chung-Hsin Lu et al. inJournal of Power Sources 97-98 (2001), pages 458-469, demonstrates therole played by the size of the lithium manganese oxide particles on thespecific capacity values and on the coulombic efficacy oflithium-manganese metal oxide particles.

In recent years, the commercial interest in metal oxides such as LiV₃O₈,V₂O₅ and LiMn₂O₄ as constituent and functional components ofelectrochemical systems has been very limited. This limitation arisesespecially from the low stability of the metal oxides in the context ofcharging-discharging cycles and the substantial losses in performanceresulting therefrom.

There is thus a need to upgrade such metal oxides considered asunstable, especially those that have good electrochemical properties,such as LiMn₂O₄ and LiV₃O₈, and moderate production costs, especially onaccount of their particular natural abundance.

SUMMARY

A first subject of the present invention consists of particlescomprising a core and a coating that covers at least part, preferably atleast 80%, even more preferentially at least 90% and most advantageously100% of the surface of said core, said particle being characterized inthat:

-   -   said core is preferably composed to at least 90% and even more        preferentially to at least 94% by weight of an acidic metal        oxide, with a pH preferably of less than 6.5, and even more        preferentially with a pH of between 3 and 6;    -   the coating of the core being based on a polymer, preferably        based on a salified polymer, whose electrochemical stability is        greater than or equal to 3.7 volts;    -   the mean thickness of the coating is preferentially between 500        nanometers and 2 micrometers and the coated core has a mean size        d₅₀ preferably of between 500 nanometers and 40 micrometers, and        even more preferentially this size is between 2 and 20        micrometers; and    -   the degree of solubility (ds), for a given time, of the metal        oxide migrating toward the electrolyte, per cycle, is less than        5 per 10 000, and this degree is preferably between 2 and 4.5        per 10 000.

According to one advantageous embodiment of the invention, the polymerused to prepare the coating is preferably chemically stable, or verystable, or even extremely stable.

Preferably, the electrochemical stability of the polymer used for thecoating is between 3.75 and 5 volts.

A preferential subfamily of particles of the invention consists ofparticles whose metal oxide is chosen from the group constituted byLiMn₂O₄, V₂O₅, LiMn_((2-x))V_(x)O₄ with x ranging from 0 to 1 limitsinclusive, V₆O₁₃ and LiV₃O₈, and the metal oxide is more preferentiallyLiV₃O₈.

According to one advantageous variant, the core of the particlecomprises from 1% to 12% and preferably from 6% to 10% by weight of acarbon preferably chosen from the group constituted by ethylene black,natural graphite, artificial graphite, Shawinigan carbon, Ketjen carbon,and mixtures of at least two thereof.

Another advantageous subfamily of particles of the invention may beconstituted by particles whose core is based on one or more salifiedpolymers, preferably based on at least one polymer salified with atleast one salt chosen from the group constituted by salts of the typeLiFSI, LiTFSI, LiBETI, LiDCTA, LiBF₄ and LiPF₆; and 10% maximum offillers preferably chosen from the group constituted by SiO₂, ZrO₂ andAl₂O₃, and mixtures of at least two thereof.

Advantageously, the polymer constituting the coating may be electricallyconductive, and may preferably be based on a polymer chosen from thegroup constituted by polyanilines, preferably from the group ofpolyanilines with an average molecular weight of greater than 1000 andpreferably between 2500 and 50 000.

The polymer constituting the coating may be electrically nonconductiveand may be advantageously chosen from the group constituted bynonconductive polymers of multibranch type.

Preferably, the electrically nonconductive polymer contains at least 3branches, and is even more preferentially of 4-branch type, such asthose described in the international patent application published onJul. 31, 2003 under WO 03/063287 (and more particularly on pages 5, 8and 9), filed in the name of Hydro-Quebec, and also in columns 1 and 2of American patent U.S. Pat. No. 6,190,804 (Ishiko et al.) and whichhave acrylate (preferably methacrylate) and alkoxy (preferably alkoxycontaining from 1 to 8 carbon atoms, even more preferentially methoxy orethoxy), or vinyl hybrid end groups.

Advantageously, the metal oxide may be a (50:50) mixture of LiV₃O₈ andV₂O₅.

Another advantageous subfamily of particles of the invention may beconstituted by particles comprising an LiV₃O₈ metal oxide core 5 micronsin size, covered over 80% of its surface with a coating constituted bythe polymer of 4-branch type and with a mean thickness of between 10nanometers and 5 micrometers and preferably between 15 nanometers and 2micrometers, characterized by a ds of less than 5%.

Another advantageous subfamily of particles of the invention may beconstituted by particles comprising a V₂O₅ metal oxide core 5micrometers in size, covered over 80% of its surface with a coatingconstituted by the 4-branch polymer and with a mean thickness of between10 nanometers and 5 micrometers and preferably between 15 nanometers and2 micrometers, characterized by a ds of less than 4%.

A second subject of the present invention consists of a process forpreparing a homogeneous mixture of particles according to the firstsubject of the invention.

The preparation may advantageously be performed by mixing the polymerand a metal oxide, via the dry route without any addition of solvent,preferably in weight proportions of from 10% to 90% and preferably from40% to 80% for each of the constituents of the mixture, the amount ofmetal oxide present in the mixture preferably being greater than that ofthe polymer.

The preparation may also be performed by homogeneous mixing of particlesaccording to the first subject of the invention, in which the mixing isperformed:

-   -   by preparing a mixture of the polymer and of a metal oxide,        preferably in weight proportions of from 10% to 90% and        preferably from 40% to 80% for each of the constituents of the        mixture; preferably, the amount of metal oxide present in the        mixture is greater than that of the polymer; and    -   with addition to the solvent of at least one solvent chosen from        the group constituted by acetone, acetonitrile, toluene, MEK,        NMP or mixtures of at least two thereof; preferably, the solvent        used represents by volume from 10% to 80% and more        preferentially from 20% to 70% of the total volume of the        solvent and of the mixture.

Advantageously, the mixing may be performed by ball milling, sandmilling, HEBM (Hot Electron Bolometer Mixer), mechanofusion, in anAgglomaster or Nobita® mixer, or by using at least two of thesetechniques, preferably at a temperature of between 10 and 40° C., andadvantageously in the presence of an inert gas chosen from the groupconstituted by nitrogen, argon or dry air.

A third subject of the present invention consists of electrodesconstituted by an electrode support, said support preferably being madeof a metallic material or of a conductive plastic material, and at leastpartially covered, preferably homogeneously, with a mixture constitutedby at least 40% and preferably from 50% to 80% by weight of particlesdefined in the first subject of the invention or obtained by one of theprocesses defined in the second subject of the invention.

Preferably, in the electrodes of the invention, at least one polymer isa binder for said electrode by creating bridges between the electrodesupport, the metal oxide-based particles and the polymer-based coating.

Advantageously, the binding polymer may be a mixture of a coatingpolymer of high stability and of binding nature and of a binding polymer(polymer that binds the particles in the cathode) other than the polymerpresent in the coating.

Preferably, the binding polymer may consist solely of a coating polymerof high electrochemical stability.

According to one advantageous embodiment of the invention, theelectrodes may comprise at least one polymer containing at least onelithium salt and at least one carbon with a specific surface area ofgreater than or equal to 1 m²/g, preferably, with a specific surfacearea of greater than 50 m²/g.

Preferably, the (polymer-oxide-salt-carbon) mixture is prepared withoutaddition solvent, advantageously by using the doctor blade method and/orby extrusion.

According to another advantageous embodiment, the(polymer-oxide-salt-carbon) mixture is prepared with addition of asolvent preferably chosen from the group constituted by acetone,acetonitrile, toluene, MEK, VC, DEC, DMC, EMC, DME or mixtures of atleast two thereof, preferably by using the doctor blade method and/or byextrusion.

Advantageously, the composition of the polymer may represent from 1% to70% by weight relative to the total weight of the(polymer+salt+oxide+carbon) mixture.

Electrodes in which the composition of the carbon represents from 1% to10% by weight relative to the total weight of the(polymer+salt+oxide+carbon) mixture are particularly advantageous.

Preferably, the concentration of the salt, present in the(polymer-oxide-salt-carbon) mixture, and expressed relative to thepolymer, is between 0.1 M and 3 M and preferably between 0.7 M and 2 M.

Another family of electrodes of the invention may be constituted byelectrodes in which the carbon, present in the(polymer-oxide-salt-carbon) mixture is a mixture of a first carbon ofgraphite nature with a specific surface area of less than 50 m²/g and ofa second carbon of non-graphite type with a large surface area, thespecific surface area of which is greater than 50 m²/g; the specificsurface area is measured according to the BET method.

Advantageously, the carbon present in the electrode is of VGCF carbonfiber, Ex mesophase or PAN (polyacronitrile) type.

Preferentially, in the electrode, the salt is dissolved in the polymerand is chosen from the group constituted by LiFSI, LiTFSI, LiBETI andLiPF₆ and mixtures of at least two thereof.

A fourth subject of the present invention consists of processes forpreparing one of the electrodes defined in the third subject of thepresent invention.

According to one advantageous embodiment, an oxide-polymer-salt-carbonliquid mixture may be spread onto a current collector of metal type byextrusion or with a doctor blade, slot die or coma.

According to one advantageous embodiment, the polymer may be of thefour-branch type preferably with at least two branches capable of givingrise to crosslinking, and it is converted into a polymer matrix,optionally in the presence of an organic solvent, by crosslinking afterspreading the mixture onto the electrode support.

The crosslinking may be advantageously performed without addition acrosslinking agent other than the metal oxide.

According to another advantageous variant, the polymer may be of EGtype, preferably with at least two branches capable of giving rise tocrosslinking, and it is converted into a polymer matrix, optionally inthe presence of an organic solvent, by crosslinking after spreading themixture onto the electrode support.

A fifth subject of the present invention consists of a process forpreparing electrochemical generators comprising at least one anode, atleast one cathode and an electrolyte, in which at least one of theelectrodes is as defined in the third subject of the present patentapplication or as obtained by one of the processes defined in the fourthsubject of the present invention.

Advantageously, these processes may be applied to the preparation ofelectrochemical generators of the lithium generator type and the spreadcathode may be introduced into said lithium generator with a dry polymeras electrolyte, the battery not containing any liquid solvent.

According to one advantageous mode, these processes may be applied tothe preparation of electrochemical generators in which the electrolyteis constituted of the same material as the binder and as the coating.

Advantageously, the process may be used for the preparation ofelectrochemical generators in which the electrolyte:

-   -   is constituted of a material other than that which constitutes        the binder and/or the coating; and/or    -   also acts as separator and is constituted by a dry polymer with        an electrochemical stability of greater than 3.7 volts; and/or    -   also acts as separator and is constituted by a dry polymer with        an electrochemical stability of less than 3.7 volts.

Advantageously, the process may be used for the preparation ofelectrochemical generators in which:

-   -   the anode is of lithium or lithium alloy or carbon, graphite,        carbon fiber, Li₄Ti₅O₁₂ or WO₂ type, preferably lithium metal or        slightly alloyed lithium; and/or    -   the lithium is alloyed with Al, Sn, carbon, Si or Mg and the        content of alloyed metals is greater than 50 ppm and preferably        greater than 500 ppm.

A sixth subject of the present invention consists of electrochemicalgenerators obtained by performing one of the processes defined in thefifth subject of the present invention.

Another advantageous family of generators of the invention may consistof those comprising particles as defined in the first subject of theinvention or as obtained by performing a process according to the secondsubject of the invention.

A seventh subject of the present invention consists of the use of agenerator as defined in the sixth subject of the present invention, inan electrical vehicle, in a hybrid vehicle, in telecommunications, inUPSs and in electrochromic devices.

An eighth subject of the present invention consists of processes forreducing the solubility of metal oxides in electrochemical systems,which consists in increasing the pH of the oxide, preferentially byselecting the nature and the amount of the carbon mixed with the oxideparticles, more particularly by coating the oxide particles with apolymer, based on PEO, polyacrylonitrile, PMMA and/or PVC dissolved in asolvent (acetonitrile, water, acetone, methanol, etc) and then by dryingthe composition and carbonizing it at a temperature of about 600-700°C., under an inert atmosphere for 8-12 hours; the amount and type of thepolymer used being linked to the residual content of carbon present atthe surface of the oxide particles, and the mixing of the oxide and thepolymer solution possibly being advantageously performed with a Jarmill, a bar mill or a paint mixer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the dissolution phenomenon observed for smalluncoated particles based on lithium vanadium oxide and of formulaLiV₃O₈.

FIG. 2 illustrates the phenomenon of dissolution of uncoated largeparticles based on lithium vanadium oxide.

FIG. 3 illustrates the general mechanism of formation of dendrites,during the dissolution of an acidic metal oxide in an electrochemicalsystem of solid polymeric lithium battery type.

FIG. 4 shows the behavior of an electrode according to the invention, inwhich the metal oxide particles have been coated with a stable polymerthat slows down the dissolution of the vanadium in the electrode.

FIG. 5 illustrates the device used for detecting the metal oxideoxidation current and for quantifying the operating voltage limit of thepolymer.

FIG. 6 gives the results, in the form of curves, for the measurement ofthe stability of LiV₃O₈ metal oxide particles coated, respectively, witha polyether polymer with a molecular weight MW of 70 000 in the firstcase, and with a 4-branch polymer, in the second case.

FIG. 7 shows the change in chemical stability of the polyether polymerwith a molecular weight MW of 70 000 for a 1-day, 3-day and then 1-weekand 2-week heat treatment.

FIG. 8 shows the change in chemical stability of the polymer ERM-4B,which is a four-branch macromonomer with an average molecular weight MWof 10 000 and a viscosity of 3.5 Pa·sec at 25° C. and characterized by anumber of acrylate functions per molecule which is 3, for a heattreatment lasting 3 days, the various curves corresponding to stabilitymeasurements taken for a 1-day, 3-day and then 1-week and 2-week heattreatment and to the observation of crosslinking.

FIG. 9 shows the change in chemical stability of the polymer EG-2500sold under the brand name Elexcel, which is a polyoxyalkylene glycolacrylate hyperbranched macromonomer in which the number of acryl groupsper molecule is 1.5, with an average molecular weight MW of 2500, andwhich is characterized by a viscosity of 0.3 Pa·sec at 25° C., thevarious curves corresponding to stability measurements taken for a1-day, 3-day, and then 1-week and 2-week heat treatment, and also to theobservation of crosslinking.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used in the context of this specification, the term “salifiedpolymer” refers to a polymer that contains a salt such that the polymeris salified by at least one salt chosen from the group constituted bysalts of the type LiFSI, LiTFSI, LiBETI, LiDCTA, LiBF₄ and LiPF₆.

As used in the context of this specification, the term “electrochemicalstability” corresponds to the window of stability of the polymer;outside this window, the polymer degrades.

As used in the context of this specification, the term “3-branchpolymers” relates, as illustrated in the document “Relationship betweenStructural Factor of Gel Electrolyte and Characteristics of Electrolyteand Lithium-ion Polymer Battery Performances” by Hiroe Nakagawa et al.,The 44th Symposium in Japan, Nov. 4-6, 2003, abstract 3D26, tothree-branch polymers which have the form of a 3-branch comb. The 3substantially parallel branches of these polymers are preferablyattached to the center and to the two ends of a small backbone,preferably containing 3 atoms, preferably 3 carbon atoms, in the chain.

In the case of a chain containing 3 carbon atoms, each of these atoms isconnected to a branch.

Among these 3-branch polymers, and in the context of the presentinvention, the ones that are preferred are those with an averagemolecular weight (MW) ranging from 1000 to 1 000 000 and even morepreferentially those whose average molecular weight ranges from 5000 to100 000.

As used in the context of this specification, the term “four-branchpolymers” is related to the abovementioned patent application WO03/063287, which is incorporated by reference into the present patentapplication, which describes a preferential family of four-branchpolymers.

Such polymers have the form of a 4-branch comb. The 4 substantiallyparallel branches of these polymers are attached, respectively, betweenthe two ends (preferably attached symmetrically to the chain) and to thetwo ends of a small chain, preferably constituted by a chain comprising4 atoms, which are preferably 4 carbon atoms.

In the case of a chain containing 4 carbon atoms, each atom is connectedto a branch.

Such polymers preferably have hybrid end groups, more preferentiallyacrylate (preferably methacrylate) and alkoxy (preferably alkoxy withfrom 1 to 8 carbon atoms, even more preferentially methoxy or ethoxy),or vinyl hybrid end groups, at least one branch of said four-branchpolymer (and preferably at least two branches) being capable of givingrise to crosslinking.

Preferably, the four-branch polymer is one of those defined in columns 1and 2 of the abovementioned American patent U.S. Pat. No. 6,190,804,which is incorporated by reference into the present patent application.

This polymer is preferentially a star polymer of polyether type whichcontains at least four branches having end groups containing thefollowing functions: acrylate or methacrylate and alkoxy, allyloxyand/or vinyloxy, of which at least one, and preferably of which at leasttwo, of these functions are active to allow crosslinking.

Other families of polyethers whose molecular mass is greater than orequal to 30 000 are advantageously used in the context of the presentinvention.

According to one preferential embodiment of the present invention, the4-branch polymer is a tetrafunctional polymer preferably with a highmolecular weight, corresponding to formula (I):

in which R¹ and R² each represent a hydrogen atom or a lower alkyl(preferably from 1 to 7 carbon atoms); R³ represents a hydrogen atom ora methyl group; m and n each represent an integer greater than or equalto 0; in each high molecular weight chain, m+n>35; and each of thegroups R¹, R² and R³ and each of the parameters m and n may be identicalor different in the 4 high molecular weight chains.

Among these four-branch polymers, those with an average molecular weightof between 1000 and 1 000 000 and even more preferentially those with anaverage molecular weight ranging from 5000 to 100 000 are particularlyadvantageous.

According to another preferential mode, polyethers of star type of atleast four branches with a hybrid end group (acrylate or methacrylateand alkoxy, allyloxy or vinyloxy) are selected. The stability voltagethereof is markedly greater than 4.

In addition, the vinyl polymers of EG type and more particularly thosedescribed in the patent application published under EP-A-1 249 461(Wendel et al.), which is incorporated by reference into the presentpatent application, are of particular interest as protective material.Among these polymers, those whose average molecular weight ranges from600 to 2500 are particularly advantageous.

Polymers of this family may advantageously be obtained by reactingethylene oxide and 2,3-epoxy-1-propanol with the starting material, orby reacting 2,3-epoxy-1-propanol with ethylene glycol as startingmaterial to produce a polymer compound. This step is followed by theintroduction of polymerizable and/or nonpolymerizable functional groupsat each end of a backbone and side chains into the resulting polymercompound.

Compounds containing one or more active hydrogen and alkoxide residuesmay also be used as starting materials.

Examples of active hydrogen residues for the compound containing one ormore active hydrogen residues include the group of hydroxyls, preferablycontaining from 1 to 5 active hydrogen residues. Specific examples ofcompounds containing one or more active hydrogen residues includetriethylene glycol monomethyl ether, ethylene glycol, glycerol,diglycerine and penta-erythritol, and derivatives thereof.

Specific examples of alkoxides also include NaOCH₃ and t-BuOK, andderivatives thereof.

The polyether polymer compounds of the invention have the structuralunit represented by formula (1) and also the structural unit representedby formula (2) and/or the structural unit represented by formula (3).The number of structural units represented by formula (1) in a moleculeis from 1 to 22 800, more advantageously from 5 to 11 400 and even moreadvantageously from 10 to 5700. The number of structural units offormula (2) or (3) (but when both are included, it is the total number)is from 1 to 13 600, more advantageously from 5 to 6800 and even moreadvantageously from 10 to 3400 and also in a molecule.

Examples of polymerizable functional groups introduced at each molecularend include (meth)acrylate residues, allyl groups and vinyl groups, andexamples of nonpolymerizable functional groups include alkyl groups orfunctional groups comprising boron atoms.

As alkyl groups above, alkyl groups containing from 1 to 6 carbon atomsare advantageous, those containing from 1 to 4 carbon atoms are moreadvantageous, and methyl groups are especially advantageous.

Examples of functional groups comprising boron atoms include thoserepresented by formula (4) or (5) below:

R¹¹ and R¹² in formula (4) and R²¹, R²² and R²³ in formula (5) may beidentical or different, and each represents a hydrogen, halogen, alkyl,alkoxy, aryl, alkenyl, alkynyl, aralkyl, cycloalkyl, cyano, hydroxyl,formyl, aryloxy, alkylthio, arylthio, acyloxy, sulfonyloxy, amino,alkylamino, arylamino, carbonamino, oxysulfonylamino, sulfonamide,oxycarbonylamino, ureido, acyl, oxycarbonyl, carbamoyl, sulfonyl,sulfinyl, oxysulfonyl, sulfamoyl, carboxylate, sulfonate, phosphonate,heterocyclic, —B(R^(a)) (R^(b)), —OB(R^(a)) (R^(b)) orOSi(R^(a))(R^(b))(R^(c)). (R^(a)), (R^(b)) and (R^(c)) each represent ahydrogen, halogen, alkyl, alkoxy, aryl, alkenyl, alkynyl, aralkyl,cycloalkyl, cyano, hydroxyl, formyl, aryloxy, alkylthio, arylthio,acyloxy, sulfonyloxy, amino, alkylamino, arylamino, carbonamino,oxysulfonylamino, sulfonamide, oxycarbonylamino, ureido, acyl,oxycarbonyl, carbamoyl, sulfonyl, sulfinyl, oxysulfonyl, sulfamoyl,carboxylate, sulfonate, phosphonate, heterocyclic or derivativesthereof. R¹¹ and R¹² in formula (4) and R²¹, R²² and R²³ in formula (5)may be linked together to form a ring, and the ring may bearsubstituents. Each group may also be substituted with substitutablegroups. Furthermore, X⁺ in formula (5) represents an alkali metal ion,and is advantageously a lithium ion.

The ends of the molecular chains in the polyether polymer may all bepolymerizable functional groups, nonpolymerizable functional groups, ormay include both.

The average molecular weight (MW) of this type of polyether polymercompound is not especially limited, but it is usually from about 500 to2 million and advantageously from about 1000 to 1.5 million.

The polymers of these preferential families are moreover advantageouslychosen from polymers that may be crosslinked by ultraviolet, infrared,heat treatment and/or an electron beam (“e-beam”).

According to the invention, it has been discovered, unexpectedly, thatthe acidic or basic nature of a metal oxide greatly influences itsstability. Thus, soluble oxides such as LiV₃O₈ and V₂O₅ generally proveto have acidic pHs, whereas insoluble oxides such as LiFePO₄ and LiCoO₂prove to have basic pHs.

Table I shows a few pH values for the metal oxides LiV₃O₈ and LiCoO₂, inthe presence of variable amounts of carbon, mixed with the oxide. The pHwas measured using the method defined below.

Table I Ketjen Carbon (% W) (W %) Sampling Test Oxide pH = 10.6 pH = 8.2time pH Color 1 LiV₃O₈ 5 30 4.52 Yellow 2 LiV₃O₈ 3.75 30 4.44 Yellow 3LiV₃O₈ 2.5 2.5 30 5.5 Yellow 4 LiFePO₄ 5 30 10.1 Colorless 5 LiFePO₄3.75 30 10.06 Colorless 6 LiFePO₄ 2.5 2.5 30 10.04 Colorless 7 LiFePO₄30 9.9 Colorless 8 LiCoO₂ 30 8.6 Colorless

According to the invention, it has also been discovered that coatingwith a particular polymer makes it possible very substantially toimprove the stability of metal oxide particles at acidic pH.

Specifically, the difficulties encountered with soluble metal oxideswere solved by the method described below, which is in accordance withthe main concept of the invention, which consists in coating the stablemetal oxide particle with a polymer of particular nature.

Thus, for example, the LiV₃O₈ particles are protected by coating themwith a polymer that is chemically and/or electrochemically stable in itselectrochemical window, i.e. with an electrochemical stability ofgreater than or equal to 3.7 volts.

The polymers used to prepare the coating are preferably of polyethertype, more preferentially of multibranch or hyperbranched type, inparticular those synthesized by the company DKS and described in patentU.S. Pat. No. 6,190,804, which is incorporated into the present patentapplication by reference.

These polymers are preferably salified with at least one salt of LiFSI,LiTFSI, LiBETI, LiBF₄ or LiPF₆ type. A mixture (polymer-with at leastone salt) that is liquid at room temperature is preferably used to coatthe oxide particles.

According to one advantageous embodiment of the invention, the metaloxide particles are coated via one of the following methods or via acombination of at least two of the methods: ball milling; jar milling;HEBM (High Energy Ball Milling); mechanofusion; using the Nobulta devicesold by the company Hosokawa, Japan; Aggolmaster; and pebble mill.

The coating of the oxide may be performed via several methods, with orwithout solvent.

The salified polymer coating the oxide particles has numerous roles. Itsfirst role is that of ensuring the nondissolution of the metal oxide,such as LiV₃O₈. Its second role is that of binder, between the oxideparticles present in the cathode spreading solution, for example on analuminum support. The third role is that of ensuring the ionconductivity of the salt, between the particles, and across theseparator.

The following parameters, relating to the nature of the polymer used toprepare the coating, play an important role in the stability, i.e. inthe low dissolution of the acidic metal oxide:

-   -   the electrochemical stability;    -   the chemical stability of the polymer; and    -   the conductive or nonconductive nature.

The various methods used for quantifying the various parameters of thepolymers used to prepare the metal oxide particle coating and toevaluate the stability of the particles of the invention are definedbelow.

Method for Measuring the pH of the Metal Oxide

The pH of the metal oxide represents the value measured, using astandard glass electrode, in an aqueous solution of the oxide obtainedby dissolving, under standard temperature and pressure conditions, 0.15grams of the oxide in 10 ml of water. The solution is left to stand forone week under standard temperature conditions, and is then stirred justbefore taking the measurement with an Oakton 2100 series machine, soldby the company Oakton. In the context of the present invention, anysample whose pH is less than 7 is classified as being an acidic metaloxide.

It is noted that the nature and the percentage of the carbon present inthe mixture (carbon-metal oxide) has an influence on the pH value; thisvalue differs more or less substantially from that of the metal oxidealone. This parameter may be used to improve the stability of the metaloxides.

In order to reduce the solubility and thus to increase the pH of theoxide, the following particular method for coating oxide particles withcarbon is preferred. The oxide powder is coated with a polymer, based onPEO, polyacrylonitrile, PMMA and/or PVC dissolved in a solvent(acetonitrile, water, acetone, methanol, etc.). Next, the composition isdried and then carbonized at a temperature of about 600-700° C. under aninert atmosphere for 8-12 hours. The amount of polymer used is relatedto the residual content of carbon present at the surface of the oxideparticles. The mixture of oxide and polymer solution may be prepared ina jar mill, a ball mill or a paint mixer.

It was demonstrated that the soluble oxides generally have acidic pHs,whereas the insoluble oxides such as LiFePO₄ and LiCoO₂ have basic pHs.

It was also discovered that the pronounced instability observed for themetal oxides of acidic nature, present in electrochemical systems,manifests itself by a migration of the soluble species to the anodewhile crossing the electrolyte and the passivation films positioned onthe anode.

This negative phenomenon of dissolution of the metal oxide and offormation of dendrites considerably reduces the physicochemicalproperties of the passivation films. This phenomenon is visualized inFIG. 3, which shows the functioning of the cell and the diagram ofuncontaminated lithium metal. This conclusion is directly derived fromexperimental XPS (X-Ray-Photoelectron Spectroscopy) results, afterdissolution of vanadium, which migrates through the solid polymerelectrolyte (SPE).

FIG. 3 especially illustrates the general mechanism of formation ofdendrites, during the dissolution of an acidic metal oxide in anelectrochemical system of solid polymer lithium battery type. SPE meansSolid Polymer Electrolyte and SEI means Solid Electrolyte Interface(passivation film). The metal oxide particles, initially present only inthe cathode, substantially decreased in size in the course of the first50 cycles and migrated partially into the SPE and into the lithiumanode. After 100 cycles, dendrites of the oxide formed. These dendritesextend from the lithium metal to the lithium separator, passing throughthe electrolyte. The references used in FIG. 3 are: 2 for lithium(negative), 4 for SPE (Solid Polymer Electrolyte—separator), 6 forcarbon, 8 for vanadium oxide, 10 for Li₂O, 12 for Li₂CO₃, 14 fordendrite and 16 for cathode.

A metal of the soluble oxide reacts with the lithium of the passivationfilm to make it electrically conductive and increases its thickness. Theformation of dendrites may result from the piercing of the SPE.Furthermore, when the dendrites reach a certain size, they may touch thecathode, thus creating a short circuit which kills the battery.

The measurements taken with a lithium polymer battery of laboratory typewith a surface area of 4 cm² show that, during cycling, in particular ata high current above C/1 (discharge in 1 hour), no dissolution takesplace for the insoluble oxides of basic nature, whereas, at the samecurrent, the soluble oxides dissolve.

Method for Measuring the Electrochemical Stability of the Metal Oxide

This stability is measured by the method developed by the Applicantusing the device shown in FIG. 3. A Mac cell is used in potentiostatmode to characterize the electrochemical stability of the materials.This is a slow cyclic voltammetry test. It is considered, when the sweepspeed is 10 mV/h, that the material is always in its thermodynamicallystable state. The current peaks generated are thus in regions in whichthere is no electrochemical activity. Degradation of the material takesplace in regions in which there is no electrochemical activity of thematerial. The degradation may intensify, as a function of the voltage,to form an irreversible electrochemical wall.

A curve of degradation of the polymer is shown in FIG. 6. The start ofthe oxidation wall expresses the stability limit of the polymer. It isby virtue of the carbon electrode of high specific surface area, such asKetjen or Shawinigan, that the electrochemical stability may be easilymeasured and the degradation of the polymer detected.

Method for Measuring the Electrochemical Stability of a Polymer

To detect the limit of the operating voltage of the polymer with theoxidation current, use is made of the electrochemical method that isalready used in the international patent application published under WO2003/063 287 (which is incorporated into the present patent applicationby reference), in the name of Hydro-Quebec, and it is performed usingthe device shown in FIG. 5.

FIG. 5 illustrates the device used for detecting the oxidation currentof a metal oxide and for quantifying the limit of the operating voltageof the polymer. The references used in FIG. 5 are: 28 for lithium, 30for SPE, 32 for Shawinigan carbon and 34 for binder (4-branch polymer).

The cathode used for the measurements is a composite of polymer and ofcarbon of high surface area spread on an aluminum current collector. Byvirtue of the surface area developed by the carbon, this material actsas a detector and can detect any oxidation current with an intensity aslow as about 2 microamperes. The electrolyte in solid form or in liquidform, saturated in a membrane of microporous polyolefin type, forinstance Celgard®, is stable at high voltage.

The anode is composed of lithium metal, which serves as referenceelectrode and as counterelectrode.

The electrochemical method used is slow cyclic voltammetry performedwith a sweep speed of 10 mV/h. This method illustrates the oxidationcurrent as a function of the voltage: each time the current approacheszero, the operating voltage of the polymer is stable.

The value of the electrochemical stability is defined as the value ofthe voltage applied to the system when the abrupt change in thecurrent/voltage is observed (I function of V).

Model of the Chemical Stability as a Preliminary Test of theElectrochemical Stability of the Coating Polymer

The stability of the polymer used as electrolyte in the cathode isdetermined by preparing a sample of spreading solution containing theoxide, the polymer, the salt and the spreading solvent. A sample of thissolution is taken (0.3 gram) and placed in a 40 ml sample bottle, andthe solvent is then evaporated off by placing the various samples undervacuum overnight. The sample bottles are sealed, under an argonatmosphere, and then transferred into an oven for treatment at 80° C.Monitoring of the stability of the polymer as a function of time wasperformed by dissolving the cathode in THF, followed by injecting itafter filtration into a GPC system in order to determine the molecularmass and the polydispersity of the extracted polymer. These results aresubsequently compared with the results for a polymer that has notundergone any heat treatment.

The results are given in FIGS. 7 to 9, in the form of comparative GPCcurves. FIG. 7 shows the change in the molecular mass of a standardpolymer as a function of time, after heat treatment. This figure showsthat after less than two hours, at 80° C., in the presence of vanadiumoxide at an OCV (open circuit voltage) of 3.55 volts, the polymercommences significant degradation. This degradation continues until thepolymer is totally degraded. This step is reached in less than 2 days.

FIGS. 8 and 9 show the change in the molecular mass of novel polymers(ERM-4B and EG-2500, respectively) under the same degradationconditions. These figures demonstrate, for these two polymers, that thesamples of extracted cathodes remain stable over a period of more than 2hours, at 80° C. Thereafter, a decrease in the level of extraction isobserved, although the molecular mass and the polydispersity of theextracted material remain stable. After more than 3 days ERM-4 branch(also known as 4-branch) and one day, respectively, for EG-2500, thepolymers are totally crosslinked and show no sign of degradation. Onlythe by-products associated with the crosslinking reaction are thenextracted and analyzed by GPC.

Via these stability analyses, performed at several temperatures, it ispossible to conclude that the novel polymers of the type ERM-4B orEG-2500 are markedly more stable to oxidation than a vanadium oxide, andthat over time it is possible to crosslink these electrolytes in situ,in the cathode, and that this network remains stable over several weeksat high temperature (80 or 110° C.).

In conclusion, it will be considered that a polymer is chemically stableif the decrease in its molecular weight, after having been subjected toa heat treatment at 80° C. for 1 day, has not decreased by more than10%.

It is considered, under these conditions, that a polymer is very stableif the decrease in its molecular weight is less than 5% under the sameconditions.

A polymer whose decrease in molecular weight, after having beensubjected to a heat treatment at 80° C. for 3 weeks, has not decreasedby more than 5% is considered as extremely stable.

The chemically stable polymers, preferentially the moderately stablepolymers and more preferentially those that are classified, according tothis test, as very stable are advantageously used in the context of thepresent invention to constitute the coating on the soluble metal oxideparticles.

Definition of the Degree of Solubility of a Metal Oxide Present in theCathode of an Electrochemical System

In the context of the present invention, the degree of solubility (ds)of the metal oxide, at a given time, refers to the percentage of metaloxide particles initially present in the cathode, which migrate to theelectrolyte and to the anode.

An example of implementation of the method for the measurement of thedegree of solubility of vanadium oxide and of the method for calculatingthe degree of dissolution of vanadium is given below.

A view in cross section of a lithium battery (lithium/SPE/cathode) isobserved under a microscope before cycling and after cycling. Beforecycling—by means of EDX (energy-dispersive X-ray) coupled to a scanningelectron microscope that maps the surface area of the vanadium elementin the cathode. This surface area is considered as the 100% referenceand, when the vanadium map is analyzed on the separator (SPE) andlithium, no trace or area of vanadium is detected. This is referred toas 0% dissolved vanadium.

After cycling—the mapping area for vanadium on SPE and lithium is 5%,i.e. before cycling, the vanadium map of the cathode corresponds to anarea of 15 cm×15 cm=225 cm². After cycling, the surface area of vanadiumfound on the SPE and the lithium surface area is 11.5 cm².

This count makes it possible to evaluate the vanadium content, which isequivalent to 5% of dissolved vanadium.

Definition of the Criterion of Conductivity and/or of Nonconductivity ofa Polymer

The electrical conductivity of a substance, also known as the capacityof a surface to conduct an electrical current, is defined as being theinverse of the resistivity: σ=1/p. As the intensity of the electricalfield in the material is expressed by the relationship E=V/L, Ohm's lawmay be written in terms of current density by the formula J=σE.Conductive metals are considered as those for which a σ>10⁵ (Ω·m)⁻¹.Semiconductive materials are considered as being those that satisfy therelationship: 10⁻⁶<σ<10⁵ (Ω·m)⁻¹. Insulating materials are considered asbeing those that satisfy the relationship σ<10⁻⁶(Ω·m)⁻¹.

In the context of the present invention, polymers with a conductivity ofgreater than 10⁻⁵ (Ω·m)⁻¹ are classified as conductive polymers, andpolymers with a conductivity of less than or equal to 10⁻⁶ (Ω·m)⁻¹ areclassified as nonconductive polymers.

Parameters for Manufacturing Electrochemical Generators IncorporatingCoated Metal Oxide Particles of the Invention

The generator or electrochemical battery is formed from at least 3 filmsacting, respectively, as anode, electrolyte and cathode.

Anode—this is a film of lithium or of lithium alloy, or of lithiumcarbon. Preferably, the film is lithium.

Electrolyte (SPE)—this is a solvent-free dry polymer film. Its naturedepends on the oxide of the cathode.

For Oxides Operating from 1 to 3.6 Volts

This case corresponds to the voltage associated with the potential athalf-discharge/100 hours, the operating voltage of the oxide is relatedto the potential at half-discharge in C/100 (discharge in 100 hours).

By way of example: Li₄Ti₅O₁₂ (1.5 volts), LiV₃O₈ (2.55 volts). In thiscase, the SPE may be of the same nature as the coating polymer, or ofdifferent nature, i.e. characterized by a stability of less than orequal to 3.7 volts.

For the Oxides Operating from 3 to 5 Volts

In this case, the SPE should preferably be of the same nature as thepolymer coating the oxide: for example LiCoO₂ (3.6 volts) and/or LiFePO₄(3.5 volts).

Cathode—it is formed from a soluble or insoluble oxide; preferably, theoxide is soluble with a coating of stable polymer type with anelectrochemical stability of greater than or equal to 3.7 volts.

The binder may be of the same nature as the coating for the oxide, whenthe oxide potential ranges from 3 to 5 volts.

EXAMPLES

The examples that follow are given purely as illustrations and shall notbe interpreted as constituting any limitation of the subjects of thepresent invention.

Table I above collates the parameters for preparing the particlesaccording to Examples 1 to 11 below.

Example 1

A mixture of Shawinigan carbon (0.91 gram) with a P70 polyether polymerof MW 70 000 (2.63 grams) and with an LiTFSI salt (0.78 gram) isprepared.

This mixture of 3 components is added to 27.3 ml of acetonitrile, andthe whole is homogenized in a jar mill for 24 hours.

The solution is spread onto an aluminum support 17 μm thick using thedoctor blade method. The electrode thus prepared is dried under vacuumat 90° C. for 24 hours. An electrode 37 micrometers thick is obtained.This electrode is named P70-carb1.

A mixture of Shawinigan carbon (0.73 gram) with a 4-branch polymerElexcel TA210 (2.06 grams) and with an LiTFSI salt (0.58 gram) isprepared. The 3 components of the mixture are then mixed with 28.9 ml ofacetonitrile. The mixture is then homogenized in a jar mill for 24hours.

The solution is spread onto an aluminum support using the doctor blademethod. The electrode obtained is dried under vacuum at 90° C. Thethickness of this electrode is 35 microns. This electrode is named4B-carb1.

The battery is mounted as follows:

-   -   lithium/electrolyte 1/4B−carbon 1=cell 1    -   lithium/electrolyte 2/P70−carbon 1=cell 2

The lithium is constituted by a film 55 microns thick. The electrolyte 1is of 4-branch nature with a thickness of 20 micrometers. Electrolyte 2is of P70 nature with a thickness of 20 micrometers.

The batteries Cell1 and Cell2 are placed in an oven at 80° C. andconnected to a Mac cell in potentiostat mode. Slow cyclic voltammetry isapplied to Cell1 and Cell2, with a sweep speed of 10 mV/h.

The battery Cell1 shows that the 4-branch polymer (FIG. 6) is stable, upto a voltage of 4.1 volts.

The battery Cell2 shows that the polymer P70 (FIG. 6) has a smalldegradation wave at about 3.3-3.6 volts, which is followed by a firstdegradation peak at about 3.74 volts.

Example 2 With Polymer P70

8.04 grams of LiV₃O₈, 0.43 gram of Ketjen carbon, 325 grams of polymerP70 and 0.90 gram of LiTFSI and 27 ml of acetonitrile are mixed togetherin a jar mill for 24 hours, after evaporation of the solvent. Theresults are given in FIG. 7 in the form of a comparative GPC curve. FIG.7 shows the change in molecular mass of the polymer, as a function ofthe duration of the heat treatment. This figure shows that after twohours at 80° C., in the presence of vanadium oxide at 3.55 volts, thepolymer commences significant degradation. This degradation continuesuntil the polymer is completely degraded, which is finally reached inless than 2 weeks.

Example 3 With 4-branch polymer

8 grams of LiV₃O₈, 0.42 gram of Ketjen carbon, 3.25 grams of 4-branchpolymer and 0.90 gram of LiTFSI are mixed with 27 ml of acetonitrile,for 24 hours in a jar mill.

After evaporation of the acetonitrile at 80° C., the measurements takenon the mixture of particles, which are given in FIG. 8, show the changein the molecular mass of novel polymers 4B, under the same degradationconditions as in Example 2.

The figure shows, for this polymer, that the samples of extractedcathodes remain stable for a period of more than 2 hours at 80° C.Thereafter, a decrease in the degree of extraction is observed, althoughthe molecular mass and the polydispersity of the extracted materialremain stable. After more than 3 days, the ERM-4B is totally crosslinkedand shows no sign of degradation.

Only the by-products associated with the crosslinking reaction are thenextracted and analyzed by GPC.

Example 4 With Polymer EG 2500

8 grams of LiV₃O₈, 0.43 gram of Ketjen carbon, 3.25 grams of polymerEG-2500 and 0.90 gram of LiTFSI are mixed with 27 ml of acetonitrile ina jar mill, for 24 hours.

After evaporation of the acetonitrile at 80° C., the measurements takenand given in FIG. 8 show the change in the molecular mass of the polymerEG-2500, under the same degradation conditions as in Examples 2 and 3.The figure moreover shows, for this polymer, that the samples ofextracted cathodes remain stable for a period of more than 2 hours at80° C. Thereafter, a decrease in the degree of extraction is observed,although the molecular mass and the polydispersity of the extractedmaterial remain stable. After more than one day, the polymer EG-2500 istotally crosslinked and shows no sign of degradation.

Only the by-products associated with the crosslinking reaction are thenextracted and analyzed by GPC.

Example 5 Polymer P70 with LiV₃O₈ Oxide/pH Below 7

8.04 grams of LiV₃O₈ and 0.43 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. The co-ground LiV₃O₈-carbon thus obtainedis mixed with 3.25 grams of polymer P70 and 0.904 gram of LiTFSI, towhich are added 27 ml of acetonitrile; this mixture is introduced into ametal container, ⅓ of the volume of which is occupied by the solution, ⅓by steel beads and ⅓ of the volume is free. The coating is obtained byHEBM, for 30 minutes at 25° C.

The solution is spread onto an aluminum current collector, using thedoctor blade method. The electrode is dried under vacuum for 24 hours.The electrode obtained has a thickness of 45 micrometers. The cellassembly is as follows: lithium/SPE/P70/LVO-P70.

The capacity of the cell is 5 mAh, and the battery is maintained at 80°C. for 2 weeks, in potentiostat mode at 3.6 volts.

The capacity fell by 25% (3.75 mAh), this loss of capacity beingdirectly linked to the dissolution of LiV₃O₈, whose pH is less than 7.

Example 6 4-Branch Polymer with LiV₃O₈/pH of 7

8.04 grams of LiV₃O₈ and 0.43 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. This co-ground LiV₃O₈-carbon is mixed with3.25 grams of the 4-branch polymer (Elexcel TA-210) and 0.904 gram ofLiTFSI, to which are added 24.7 ml of acetonitrile. The mixture thusobtained is introduced into a metal container, ⅓ of the volume of whichis occupied by the solution of this mixture, ⅓ by steel beads and ⅓ ofthe volume remains free. The coating is obtained by HEBM, for 30 minutesat 25° C.

The solution is spread onto an aluminum current collector, via thedoctor blade method. The electrode is dried under vacuum for 24 hours;the electrode has a thickness of 45 micrometers. The cell assembly is asfollows: lithium/SPE/4-branch/LVO-4-branch.

The capacity of the cell is 5.5 mAh, and the battery is maintained at80° C. for 2 weeks in potentiostat mode at 3.6 volts.

The capacity then measured is 5.2 mAh. The loss of capacity is 1%, whichis the limit error of the capacity. This proves that LiV₃O₈ is notdissolved.

Example 7 EG with LVO-EG

8.04 grams of LiV₃O₈ and 0.43 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. This co-ground LiV₃O₈-carbon is mixed with3.25 grams of polymer EG 2500 and 0.904 gram of LiTFSI, to which areadded 45 ml of acetonitrile; this mixture is introduced into a metalcontainer, ⅓ of the volume of which consists of the solution of thismixture, ⅓ of steel beads 6.34 mm in diameter, and ⅓ of the volumeremains free. The coating is obtained by HEBM for 30 minutes at 25° C.

The solution is spread onto an aluminum current collector via the doctorblade method. The LVO-EG electrode obtained is dried under vacuum for 24hours. The electrode has a thickness of 45 micrometers. The cellassembly is as follows: lithium/SPE-EG/-/LVO-EG.

The capacity of the cell is 5.5 mAh, and the battery is maintained at80° C. for 2 weeks, in potentiostat mode at 3.6 volts.

The capacity then fell by 25% (4.1 mAh). This loss of capacity isdirectly linked to the dissolution of LiV₃O₈, whose pH is less than 7.

Example 8 P70 with LiFePO₄/pH Above 7

8 grams of LiFePO₄ and 0.45 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. This co-ground LiFePO₄-carbon is mixedwith 3.25 grams of polymer P70 and 0.9 gram of LiTFSI, to which areadded 45 ml of acetonitrile. This mixture is introduced into a metalcontainer, ⅓ of the volume of which is filled with the solution of thismixture, ⅓ with steel beads 6.34 mm in diameter and ⅓ of the volumeremains free. The coating is obtained by HEBM for 30 minutes at 25° C.

The solution is spread onto an aluminum current collector, via thedoctor blade method. The LiFePO₄-P70 electrode obtained is dried undervacuum for 24 hours. The electrode has a thickness of 45 micrometers.The cell assembly is as follows: lithium/SPE-P70/-/LVO-P70.

The capacity of the cell is 4.5 mAh, and the battery is maintained at80° C. for 2 weeks, in potentiostat mode at 3.63 volts.

The capacity then fell by 27% (4.28 mAh). This loss of capacity isdirectly linked to the instability to oxidation of the polymer P70.

When the battery is maintained at 80° C. for 2 weeks, in potentiostatmode at 4.00 volts, the capacity falls by 51% (2.2 mAh). This loss ofcapacity is directly linked to the instability to oxidation of thepolymer P70.

Example 9 4-Branch with LiFePO₄/pH Above 7

78 grams of LiFePO₄ and 0.45 gram of Ketjen carbon are dry-mixed, bymechanofusion, for 45 minutes. This co-ground LiFePO₄-carbon is mixedwith 3.25 grams of 4-branch polymer and 0.9 gram of LiTFSI, to which areadded 45 ml of acetonitrile; this mixture is introduced into a metalcontainer, ⅓ of the volume of which is filled with the solution of themixture, ⅓ with steel beads 6.34 mm in diameter and ⅓ of the volumeremains free. The coating is obtained by HEBM, for 30 minutes at 25° C.

The solution is spread onto an aluminum current collector, via thedoctor blade method. The LiFePO₄-4B electrode is dried under vacuum for24 hours. The electrode has a thickness of 45 micrometers. The cellassembly is as follows: lithium/SPE-EG/-/LiFePO₄-4B.

The capacity of the cell is 4.5 mAh. The battery is maintained at 80° C.for 2 weeks in potentiostat mode at 3.63 volts.

The capacity remains unchanged (4.5 mAh), the maintenance of thecapacity being directly linked to the stability to oxidation of the4-branch polymer.

When the battery is maintained at 80° C. for 2 weeks, in potentiostatmode at 4.00 volts, the loss of capacity is 1% (4.45 mAh). This loss ofcapacity is directly linked to the instability to oxidation of thepolymer.

The maintenance of the capacity is directly linked to the stability tooxidation of the 4-branch polymer (4B).

Example 10 4-Branch with LiFePO₄/pH Above 7

78 grams of LiFePO₄ and 0.45 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. This co-ground LiFePO₄-carbon is mixedwith 3.25 grams of polymer EG-2500 and 0.9 gram of LiTFSI, to which areadded 45 ml of acetonitrile. This mixture is introduced into a metalcontainer, ⅓ of the volume of which is occupied by the solution of thismixture, ⅓ by steel beads 6.34 mm in diameter and ⅓ of the volumeremains free. The coating is obtained by HEBM for 30 minutes at 25° C.

The solution is spread onto an aluminum current collector, using thedoctor blade method. The LiFePO₄-4B electrode is dried under vacuum for24 hours. The electrode has a thickness of 45 micrometers. The cellassembly is as follows: lithium/SPE-EG/-/LiFePO₄-4B.

The capacity of the cell is 4.5 mAh, and the battery is maintained at80° C. for 2 weeks, in potentiostat mode at 3.63 volts.

The capacity remains unchanged (4.5 mAh). Maintenance of the capacity isdirectly linked to the stability to oxidation of the 4-branch polymer.

When the battery is maintained at 80° C. for 2 weeks, in potentiostatmode at 4.00 volts, the loss of capacity is 1% (4.45 mAh). This loss ofcapacity is directly linked to the instability to oxidation of thepolymer.

Maintenance of the capacity is directly linked to the stability tooxidation of the 4-branch polymer.

Example 11 EG2500 with LiFePO₄/pH Above 7

78 grams of LiFePO₄ and 0.45 gram of Ketjen carbon are dry-mixed bymechanofusion for 45 minutes. The co-ground LiFePO₄-carbon obtained ismixed with 3.25 grams of polymer EG-2500 and 0.9 of LiTFSI, to which areadded 45 ml of acetonitrile. The mixture thus obtained is introducedinto a metal container, ⅓ of the volume of which is filled with thesolution of this mixture, ⅓ with steel beads 6.34 mm in diameter, and ⅓of the volume remains free. The coating is obtained by HEBM for 30minutes at 25° C.

The solution is spread onto an aluminum current collector 17 μm thick,using the doctor blade method. The LiFePO₄-4B electrode is dried undervacuum for 24 hours; it has a thickness of 45 micrometers. The cellassembly is as follows: lithium/SPE-EG/-/LiFePO₄-polymer 4B.

The capacity of the cell is 4.5 mAh. The battery is maintained at 80° C.for 2 weeks, in potentiostat mode at 3.63 volts.

The capacity remains unchanged (4.5 mAh), maintenance of the capacitybeing directly linked to the stability to oxidation of the 4-branchpolymer.

When the battery is maintained at 80° C. for 2 weeks in potentiostatmode at 4.00 volts, the loss of capacity is 1% (4.45 mAh). This loss ofcapacity is directly linked to the instability to oxidation of thepolymer.

Maintenance of the capacity is directly linked to the stability tooxidation of the EG polymer.

The coated particles of the invention prove to have excellentelectrochemical properties, especially as regards the service life ofthe batteries into which they are incorporated and on account of theireconomic value.

FIG. 1 illustrates the dissolution phenomenon observed for smalluncoated particles based on lithium vanadium oxide of formula LiV₃O₈.The insertion of Li into vanadium oxide causes an increase in the volumeof the particles; originally of an average size of 5 micrometers (D50).The lithiated particles have an average size of 5.5 micrometers (D55),which corresponds to a size increase of 3 to 4%. After 50 cycles, themetal oxide particles have a smaller average size, of 3.5 micrometers.

FIG. 2 illustrates the dissolution phenomenon of large uncoatedparticles based on lithiated vanadium oxide. The insertion of Li intovanadium oxide causes an increase in the volume of the particles. Thelithiated particles have an average size of 33 micrometers. After 100cycles, the metal oxide particles have an average size of 23micrometers.

FIG. 4 shows the behavior of an electrode according to the invention, inwhich the metal oxide particles have been coated with a stable polymerthat slows down the dissolution of the vanadium in the electrode. Thereferences used in FIG. 4 are: 18 for lithium, 20 for SPE, 22 for Ketjencarbon, 24 for (4-branch polymer) coating the oxide, and 5 for oxidecore.

Although the present invention has been described with the aid ofspecific implementations, it is understood that several variations andmodifications may be applied to said implementations, and the presentinvention is directed toward covering such modifications, uses oradaptations of the present invention, following in general theprinciples of the invention and including any variation of the presentdescription that will become known or conventional in the field ofactivity in which the present invention is based, and which may beapplied to the essential elements mentioned above.

1. A particle comprising a core and a coating that covers at least part,preferably at least 80%, even more preferentially at least 90% and mostadvantageously 100% of the surface of said core, wherein: said core ispreferably composed to at least 90% and even more preferentially to atleast 94% by weight of an acidic metal oxide, with a pH preferably ofless than 6.5, and even more preferentially with a pH of between 3 and6; the coating of the core being based on a polymer, preferably based ona salified polymer, whose electrochemical stability is greater than orequal to 3.7 volts; the mean thickness of the coating is preferentiallybetween 500 nanometers and 2 micrometers and the coated core has a meansize d₅₀ preferably of between 500 nanometers and 40 micrometers, andeven more preferentially this size is between 2 and 20 micrometers; andthe degree of solubility (ds), for a given time, of the metal oxidemigrating toward the electrolyte, per cycle, is less than 5 per 10 000,and this degree is preferably between 2 and 4.5 per 10
 000. 2. Theparticle as claimed in claim 1, wherein the polymer used to prepare thecoating is chemically stable.
 3. The particle as claimed in claim 2,wherein the polymer used to prepare the coating is chemically verystable.
 4. The particle as claimed in claim 3, wherein the polymer usedto prepare the coating is chemically extremely stable.
 5. The particleas claimed in claim 4, wherein the electrochemical stability of thepolymer used for the coating is between 3.75 and 5 volts.
 6. Theparticle as claimed in claim 1, in which the metal oxide is chosen fromthe group constituted by LiMn₂O₄, V₂O₅, LiMn_((2-x))V_(x)O₄ with xranging from 0 to 1 limits inclusive, V₆O₁₃ and LiV₃O₈ and the metaloxide is more preferentially LiV₃O₈.
 7. The particle as claimed in claim1, in which the core of the particle comprises from 1% to 12% andpreferably from 6% to 10% by weight of a carbon preferably chosen fromthe group constituted by ethylene black, natural graphite, artificialgraphite, Shawinigan carbon, Ketjen carbon, and mixtures of at least twothereof.
 8. The particle as claimed in claim 1, in which the corecoating is based on: one or more salified polymers, preferably based onat least one polymer salified with at least one salt chosen from thegroup constituted by salts of the type LiFSI, LiTFSI, LiBETI, LiDCTA,LiBF₄ and LiPF₆; and 10% maximum of fillers preferably chosen from thegroup constituted by SiO₂, ZrO₂ and Al₂O₃, and mixtures of at least twothereof.
 9. The particle as claimed in claim 1, in which the polymerconstituting the coating is electrically conductive, and is preferablybased on a polymer chosen from the group constituted by polyanilines,preferably from the group of polyanilines with an average molecularweight of greater than 1000 and preferably between 2500 and 50
 000. 10.The particle as claimed in claim 1, in which the polymer constitutingthe coating is electrically nonconductive and is preferably chosen fromthe group constituted by nonconductive polymers of multibranch type. 11.The particle as claimed in claim 10, in which the electricallynonconductive polymer contains at least 3 branches, and is even morepreferentially of 4-branch type, such as those described ininternational patent application WO 03/063 287 (and more particularly onpages 5, 8 and 9), filed in the name of Hydro-Quebec, and also incolumns 1 and 2 of American patent U.S. Pat. No. 6,190,804 and whichhave acrylate (preferably methacrylate) and alkoxy (preferably alkoxycontaining from 1 to 8 carbon atoms, even more preferentially methoxy orethoxy), or vinyl hybrid end groups.
 12. The particle as claimed inclaim 1, in which the metal oxide is a (50:50) mixture of LiV₃O₈ andV₂O₅.
 13. The particle as claimed in claim 1, comprising an LiV₃08 metaloxide core 5 microns in size, covered over 80% of its surface with acoating constituted by the polymer of 4-branch type and with a meanthickness of between 10 nanometers and 5 micrometers and preferablybetween 15 nanometers and 2 micrometers, comprising a ds of less than5%.
 14. The particle as claimed in claim 1, comprising a V₂O₅ metaloxide core 5 micrometers in size, covered over 80% of its surface with acoating constituted by the polymer 4B and with a mean thickness ofbetween 10 nanometers and 5 micrometers and preferably between 15nanometers and 2 micrometers, comprising a ds of less than 4%.
 15. Aprocess for preparing a homogeneous mixture of particles as claimed inclaim 1, by preparation of a mixture of the polymer and of a metaloxide, via the dry route without any addition of solvent, preferably inweight proportions of from 10% to 90% and preferably from 40% to 80% foreach of the constituents of the mixture, the amount of metal oxidepresent in the mixture preferably being greater than that of thepolymer.
 16. A process for preparing a homogeneous mixture of particlesas claimed in claim 1, in which the mixing is performed: by preparing amixture of the polymer and of a metal oxide, preferably in weightproportions of from 10% to 90% and preferably from 40% to 80% for eachof the constituents of the mixture; preferably, the amount of metaloxide present in the mixture is greater than that of the polymer; andwith addition to the solvent of at least one solvent chosen from thegroup constituted by acetone, acetonitrile, toluene, MEK, NMP ormixtures of at least two thereof; preferably, the solvent usedrepresents by volume from 10% to 80% and more preferentially from 20% to70% of the total volume of the solvent and of the mixture
 17. Theprocess as claimed in claim 15, in which the mixing is performed by ballmilling, sand milling, HEBM, mechanofusion, in an Agglomaster or Nobita®mixer, or by using at least two of these techniques and preferably at atemperature of between 10 and 40° C., preferably in the presence of aninert gas chosen from the group constituted by nitrogen, argon or dryair.
 18. An electrode constituted by an electrode support, said supportpreferably being made of a metallic material or of a conductive plasticmaterial, and at least partially covered, preferably homogeneously, witha mixture constituted by at least 40% and preferably from 50% to 80% byweight of particles comprising a core and a coating that covers at leastpart, preferably at least 80%, even more preferentially at least 90% andmost advantageously 100% of the surface of said core, wherein: said coreis preferably composed to at least 90% and even more preferentially toat least 94% by weight of an acidic metal oxide, with a pH preferably ofless than 6.5, and even more Preferentially with a pH of between 3 and6; the coating of the core being based on a polymer, preferably based ona salified polymer, whose electrochemical stability is greater than orequal to 3.7 volts; the mean thickness of the coating is preferentiallybetween 500 nanometers and 2 micrometers and the coated core has a meansize d₅₀ preferably of between 500 nanometers and 40 micrometers, andeven more preferentially this size is between 2 and 20 micrometers; andthe degree of solubility (ds), for a given time, of the metal oxidemigrating toward the electrolyte, per cycle, is less than 5 per 10 000,and this degree is preferably between 2 and 4.5 per 10 000 or obtainedby process defined in claim
 15. 19. The electrode as claimed in claim18, in which at least one polymer is a binder for said electrode bycreating bridges between the electrode support, the metal oxide-basedparticles and the polymer-based coating
 20. The electrode as claimed inclaim 19, in which the binding polymer is a mixture of a coating polymerof high stability and of binding nature and of a polymer that ensuresbinding between the particles of the cathode and that is other than thepolymer present in the coating.
 21. The electrode as claimed in claim19, in which the binding polymer consists solely of the coating polymerof high electrochemical stability.
 22. The electrode as claimed in claim18, comprising at least one polymer containing at least one lithium saltand at least one carbon with a specific surface area of greater than orequal to 1 m²/g, preferably at least one carbon with a specific surfacearea of greater than 50 m²/g.
 23. The electrode as claimed in claim 22,in which the (polymer-oxide-salt-carbon) mixture has been preparedwithout addition of solvent, preferably by using the doctor blade methodand/or by extrusion.
 24. The electrode as claimed in claim 22, in whichthe (polymer-oxide-salt-carbon) mixture has been prepared with additionof a solvent preferably chosen from the group constituted by acetone,acetonitrile, toluene, MEK, VC, DEC, DMC, EMC, DME or mixtures of atleast two thereof, preferably by using the doctor blade method and/or byextrusion.
 25. The electrode as claimed in claim 23, in which thecomposition of the polymer represents from 1% to 70% by weight relativeto the total weight of the (polymer+salt+oxide+carbon) mixture.
 26. Theelectrode as claimed in claim 25, in which the composition of the carbonrepresents from 1% to 10% by weight relative to the total weight of the(polymer+salt+oxide+carbon) mixture.
 27. The electrode as claimed inclaim 18, in which the concentration of the salt, present in the(polymer-oxide-salt-carbon) mixture, and expressed relative to thepolymer, is between 0.1 M and 3 M and preferably between 0.7 M and 2 M.28. The electrode as claimed in claim 18, in which the carbon, presentin the (polymer-oxide-salt-carbon) mixture, is a mixture of a firstcarbon of graphite nature with a specific surface area of less than 50m²/g and of a second carbon of non-graphite type with a large surfacearea, the specific surface area of which is greater than 50 m²/g, thespecific surface area being measured according to the BET method. 29.The electrode as claimed in claim 18, in which the carbon is of VGCFcarbon fiber, Ex mesophase or PAN (polyacrylonitrile) type.
 30. Theelectrode as claimed in claim 29, in which the salt is dissolved in thepolymer and it is chosen from the group constituted by LiFSI, LiTFSI,LiBETI and LiPF₆ and mixtures of at least two thereof.
 31. A process forpreparing an electrode as claimed in claim 18, in which anoxide-polymer-salt-carbon liquid mixture is spread onto a currentcollector of metal type by extrusion or with a doctor blade, slot die orcoma.
 32. The process for preparing an electrode as claimed in claim 31,in which the polymer is of the four-branch type preferably with at leasttwo branches capable of giving rise to crosslinking, and it is convertedinto a polymer matrix, optionally in the presence of an organic solvent,by crosslinking after spreading the mixture onto the electrode support.33. The process for preparing an electrode as claimed in claim 32, inwhich the crosslinking is performed without addition of a crosslinkingagent other than the metal oxide.
 34. A process for preparing anelectrochemical generator comprising at least one anode, at least onecathode and an electrolyte, in which at least one of the electrodes isas defined in claim
 18. 35. The process for preparing an electrochemicalgenerator as claimed in claim 34, in which the electrochemical generatoris of the lithium generator type and the spread cathode is introducedinto said lithium generator with a dry polymer as electrolyte, thebattery not containing any liquid solvent.
 36. The process for preparingan electrochemical generator as claimed in claim 35, in which theelectrolyte is constituted of the same material as the binder and as thecoating.
 37. The process for preparing an electrochemical generator asclaimed in claim 35, in which the electrolyte is constituted of amaterial other than that which constitutes the binder and/or thecoating.
 38. The process for preparing an electrochemical generator asclaimed in claim 34, in which the electrolyte also acts as separator andis constituted by a dry polymer with an electrochemical stability ofgreater than 3.7 volts.
 39. The process for preparing an electrochemicalgenerator as claimed in claim 34, in which the electrolyte also acts asseparator and is constituted by a dry polymer with an electrochemicalstability of less than 3.7 volts.
 40. The process for preparing anelectrochemical generator as claimed in claim 34, in which the anode isof lithium or lithium alloy or carbon, graphite, carbon fiber, Li₄Ti₅O₁₂or WO₂ type, preferably lithium metal or slightly alloyed lithium. 41.The process for preparing an electrochemical generator as claimed inclaim 40, in which the lithium is alloyed with Al, Sn, carbon, Si or Mgand the content of alloyed metals is greater than 50 ppm and preferablygreater than 500 ppm.
 42. An electrochemical generator obtained byperforming one of the processes as claimed in claim
 31. 43. Anelectrochemical generator containing at least one constituent elementcomprising particles comprising a core and a coating that covers atleast part, preferably at least 80%, even more preferentially at least90% and most advantageously 100% of the surface of said core, wherein:said core is preferably composed to at least 90% and even morepreferentially to at least 94% by weight of an acidic metal oxide, witha pH preferably of less than 6.5, and even more preferentially with a pHof between 3 and 6; the coating of the core being based on a polymer,preferably based on a salified polymer, whose electrochemical stabilityis greater than or equal to 3.7 volts; the mean thickness of the coatingis preferentially between 500 nanometers and 2 micrometers and thecoated core has a mean size d₅₀ preferably of between 500 nanometers and40 micrometers, and even more preferentially this size is between 2 and20 micrometers; and the degree of solubility (ds), for a given time, ofthe metal oxide migrating toward the electrolyte, per cycle, is lessthan 5 per 10 000, and this degree is preferably between 2 and 4.5 per10 000 or as obtained by performing a process as claimed in claim 15.44. A generator obtained by performing one of the processes defined inclaim
 36. 45. The use of a generator as claimed in claim 44 in anelectrical vehicle, in a hybrid vehicle, in telecommunications, in UPSsand in electrochromic devices.
 46. The process for preparing anelectrode as claimed in claim 31, in which the polymer is of EG type,preferably with at least two branches capable of giving rise tocrosslinking, and said polymer is converted into a polymer matrix,optionally in the presence of an organic solvent, by crosslinking afterspreading the mixture onto the electrode support.
 47. A process forreducing the solubility of metal oxides in electrochemical systems,which consists in increasing the pH of the oxide, preferentially byselecting the nature and the amount of the carbon mixed with the oxideparticles, more particularly by coating the oxide particles with apolymer, based on PEO, polyacrylonitrile, PMMA and/or PVC dissolved in asolvent (acetonitrile, water, acetone, methanol, etc) and then by dryingthe composition and carbonizing it at a temperature of about 600-700°C., under an inert atmosphere for 8-12 hours; the amount and type of thepolymer used being linked to the residual content of carbon present atthe surface of the oxide particles, and the mixing of the oxide and thepolymer solution possibly being advantageously performed with a jarmill, a bar mill or a paint mixer.
 48. A process for preparing anelectrochemical generator comprising at least one anode, at least onecathode and an electrolyte, as obtained by one of the processes definedin claim
 31. 49. A generator obtained by performing one of the processesas defined in claim 42.